Typically access tools, such as a ring cut and slitter tools, are used to open buffer tubes in order to work on fibers disposed within a buffer tube also containing signal-carrying fibers. Tearing the buffer tube with a fingernail is also a technique used to gain access to the fiber. With these techniques, there is a risk that during access, the tool or fingernail will perturb and cause attenuation to signals carried in live fibers or cause damage to the fiber coating. Such attenuation is on the order of 0.5 dB and higher. An additional concern is that during removal of the buffer tube, the ring cut or breaking away of the buffer tube, a fiber will become damaged or will break.
In order to mitigate the risk of damage to optical fibers during the process of buffer tube tearing and removal, several patents have included concepts of including a ripcord filament within the tube, or using specialized tools to slice open a section of the buffer tube. The rip filament may be embedded in the buffer tube or encased in the buffer tube and is discussed in an Alcatel patent by Harbort entitled xe2x80x9cOptical Cable having at Least Two Separate Multiple-Fiber units each having its own Soft Plastic Envelope,xe2x80x9d U.S. Pat. No. 4,909,593. The following patents discuss different access tools used to slice sections of the buffer tube. These include xe2x80x9cMonotube Optical Fiber Cable and Method of using the Same,xe2x80x9d U.S. Pat. No. 5,172,620, xe2x80x9cMonotube Cable Fiber Access Tool,xe2x80x9d U.S. Pat. No. 5,140,751 and xe2x80x9cMethod and Tool for Accessing Optical Fibers Within a Buffer Tube,xe2x80x9d U.S. Pat. No. 5,577,150. However, in these patents, access tools are required to both initiate and propagate the tear in the buffer tube using a blade. Therefore, the risk of fiber damage or breakage and signal attenuation is not adequately addressed.
The above references also do not describe concepts on the correct fracture mechanics to make the rip system stable. An example of a conventional rip system is illustrated in FIG. 2. As shown, a ripcord is being manipulated to tear through a buffer tube. In this case, a notch cannot be initiated in the buffer tube using the ripcord. The force required to create a notch with the ripcord is greater than the force to buckle the buffer tube. The rip system is clearly unstable as indicated by the significant curling and buckling of the fiber bundle on the right-hand side of FIG. 1. This deformation of the fiber bundle contributes to the optical fiber damage and signal attenuation. As additional energy is applied in attempt to pull the ripcord through the buffer tube, the degree of buckling increases. Power loss measurements for the signal carried on an optical fiber disposed within the buffer tube of the conventional system are shown in FIG. 3.
The present invention overcomes the above deficiencies of known buffer tubes and access techniques by providing a variety buffer tube designs that are strong enough to house fiber bundles together yet permits the fibers disposed within the tube to be accessed easily with minimal attenuation. The present buffer tube designs further mitigate the risk of fiber breakage, because no sharp cutting implements need to be used. In particular, the buffer tubes of the present invention are constructed from materials having an appropriate ratio between the energy needed to deform the buffer tube versus the energy needed to tear the buffer tube, with or without a ripcord, and how this relates to reduced attenuation.
A primary feature of the invention comprises a ripcord disposed interior to the buffer tube or embedded in its wall. If a primary access method is used to open the buffer tube, the ripcord can be handled and used to promote a controlled tear in the buffer tube to allow access to several feet of fiber. The fracture energies of the buffer tube are selected so that the energy to pull the ripcord through the tube wall is less than the energy to bend the buffer tube bundle. The energy to yield of the ripcord should be greater than the energy to rip the ripcord through the buffer tube so that the ripcord does not break while attempting to remove the buffer tube.
A second feature of the invention comprises a buffer tube with mechanical properties such that its energy to initiate a tear (e.g. energy to break) and propagate a tear are less than the energy needed to deform a fiber bundle. This reduces the chance of excessive attenuation to signals carried in the optical fibers. In this aspect of the invention, no primary access tools are required to initiate the tear in the buffer tube due to mechanical and material properties of the buffer tube. In this embodiment, the ripcord need not be provided to propagate a tear.
A third feature of the invention comprises a buffer tube with stress risers in the tube walls that promote a controlled fracture. The energy to fracture the buffer tube wall is low enough so that the fibers can be easily accessed or even lower such that it does not perturb the fiber bundle and does not cause significant attenuation to live fibers. The stress risers can be in the longitudinal direction parallel to the lay of the optic fibers, perpendicular to the fiber direction or in a spiral pattern. The stress risers can be either continually or intermittently formed in the buffer tube.